The examination of isolated cells is a declared goal in science. There are numerous microfluidic chips with which one attempts to capture single cells with surface interactions in gels or force fields, for example optical tweezers, magnetic, acoustic, positive or negative dielectrophoretic interactions, in the flowing medium, in order to be able to cultivate and analyze them separately. What is disadvantageous with optical tweezers is that the cells are kept at an energy maximum which leads to a rapid temperature rise in the cell. Moreover, many methods are too weak to trap the cells in the microfluidic stream contactlessly.
Manipulation of particles by means of dielectrophoresis is a known technology and the separation and sorting of even larger cells is a main use of dielectrophoresis. With positive dielectrophoresis (pDEP) cells are drawn to the electrodes and stick to these by dielectric polarization. The cells are freed again by switching off the AC field. Positive dielectrophoresis has the disadvantage that the cells come in contact with the electrodes and are held in the area of the highest force field intensity. An increase of temperature at the electrodes (Eng. Joule heating) often kills off cells. With negative dielectrophoresis (nDep), cells are repelled from the electrodes. This can be used with a certain three-dimensional arrangement to accumulate cells in the field minimum contactlessly or to capture them in the flowing medium. In this connection, cells can be trapped in a field cage of high-frequency nDEP fields that are produced by activations of octupoles, as an example.
Microfluidic devices for dielectrophoretic separation or accumulation of cells are known. However, with the system described it is disadvantageous with the capture of a cell to agglomerate subsequent cells by nDEP repulsion and in the subsequent single-cell cultivation they adhered in large numbers in the vicinity of the cultivated cell, particularly in the same fluid stream, or were even drawn into the fluid stream going into the analysis. If there are more cells in a microfluidic channel quickly secreted substances can be exchanged from cell to cell over the short diffusion paths. Uncontrolled exchange leads to undefined environmental conditions during single-cell cultivation. Isolation and separate, contactless cultivation, however, is helpful for defined cultivation conditions.